Thyroid hormones and muscle protein turnover. The effect of thyroid-hormone deficiency and replacement in thryoidectomized and hypophysectomized rats.

We have investigated the effects of thyroidectomy, hypophysectomy and 3,3',5-tri-iodothyronine replacement on protein synthesis and degradation in skeletal muscle in vivo. Thyroidectomy resulted in a decrease in the rate of protein synthesis as a result of a loss of RNA. However, RNA activity, the rate of protein synthesis per unit of RNA, was not decreased. This was the case in both young growing rats and mature nongrowing rats. Tri-iodothyronine treatment of thyroidectomized rats increased protein synthesis by increasing RNA concentration without changes in RNA activity, and this occurred even when food intake was restricted to prevent any increase in growth. The rate of protein degradation was decreased by thyroidectomy and increased by tri-iodo-thyronine replacement in both animals fed ad libitum and food-restricted animals. Hypophysectomy decreased protein synthesis by decreasing both RNA concentration and activity. these changes were reversed by tri-iodothyronine treatment even in the presence of persistent marked hypoinsulinaemia. This indicates that tri-iodothyronine can activate athe translational phase of protein synthesis in muscle in the absence of significant quantities of insulin. However, tri-iodothyronine does not seem to be obligatory for the maintenance of normal RNA activity in muscle, since in the thyroidectomized rat, in which plasma insulin concentrations are normal, RNA activity is maintained. From a consideration of the magnitude of changes in RNA activity observed in these experiments, it would appear that alterations in rates of elongation as well as initiation are involved in the changes in RNA activity.     

Protein synthesis in the early stages of cardiac hypertrophy

Cardiac hypertrophy, induced in rats by either tri-iodothyronine or isoproterenol, administered daily for 7 days, was monitored using several parameters. Both treatments increased RNA concentrations 24 hr after the first injection, while heart weight increased following 2 injections to 46% above control after 7 days. Cardiac protein synthetic activity, as determined by the rate of peptidyl-puromycin formation, was increased by both tri-iodothyronine and isoproterenol 24 hr after a single injection, implying an increase in the number of functional ribosomes. RNA activity (the rate of peptidyl-puromycin formation per unit RNA) remained constant, suggesting that neither accelerated rates of initiation or translation nor increased activation of pre-existing, non-translating ribosomes was involved in the observed increase in protein synthetic activity. In contrast, constant infusion of [14C] tyrosine indicated no change in protein synthetic rate 24 hr after a single tri-iodothyronine injection and decreased protein synthetic rate after isoproterenol injection. It is concluded that the use of [3H]puromycin to estimate protein synthetic activity may be a more sensitive procedure for detecting early changes in protein synthesis in cardiac hypertrophy than constant isotope infusion, owing to the problems associated with determining the precise precursor pool for protein synthesis in this latter method.     

Early stimulation of rat liver microsomal protein synthesis after tri-iodothyronine injection in vivo.

In an effort to determine the physiological significance of previous studies showing stimulation of microsomal protein synthesis by thyroxine added in vitro, an early effect of tri-iodothyronine injected in vivo was sought. Tri-iodothyronine (25 micrograms/100 g) administered to euthyroid rats stimulated microsomal protein synthesis in vitro within 3--6 h. This effect occurred much earlier than the 26 h lag previously reported after tri-iodothyronine administration to hypothyroid rats. This early effect of tri-iodothyronine on protein synthesis is prevented by alpha-amanitin, suggesting that it is dependent on RNA synthesis. The failure to find a direct effect in vivo of tri-iodothyronine on translation casts doubt on the physiological significance of previous studies that have shown a direct stimulation of translation by thyroxine added in vitro.     

Muscle protein synthesis in the streptozotocin-diabetic rat. A possible role for corticosterone in the insensitivity to insulin infusion in vivo.

The effect of insulin infusion in vivo on muscle protein synthesis was investigated in rats. In 10-days-streptozotocin-diabetic rats infused in vivo with amino acids and glucose, the rate of protein synthesis per unit of RNA (RNA activity) was markedly decreased. Pre-treatment with large doses of insulin at 17 and 1 h before the infusion fully restored RNA activity to normal. Infusion of insulin for 6 h with amino acids and glucose did not restore RNA activity to normal in the diabetic rats. However, in diabetic-adrenalectomized rats similar infusions of insulin fully restored RNA activity to normal. Measurements of plasma corticosterone concentrations indicated a 50% increase in the diabetic rats. Since pre-treatment with corticosterone suppressed the stimulatory effect of insulin infusion on RNA activity in adrenalectomized rats, and since corticosterone treatment for 6 days suppressed RNA activity even though insulin concentrations were elevated, it is suggested that increased concentrations of corticosterone are responsible for the lag in response to insulin in the diabetic rat. This means that the catabolic effects of glucocorticoids must be also considered together with the catabolic effect of insulin lack in diabetes.     

Ribonucleic acid synthesis during the early action of thyroid hormones

1. The effect on RNA synthesis in rat liver of thyroidectomy and the administration of thyroid hormone, especially during its physiological latent period, was studied by determining: (a) the activity of DNA-dependent RNA polymerase in isolated nuclei; (b) the rate of synthesis of nuclear and cytoplasmic RNA in vivo; (c) polyribosomal sedimentation profiles; (d) the response of microsomes and ribonucleoprotein particles to polyuridylic acid; (e) the effect of inhibitors of RNA and protein synthesis on the biological activity of hormones. 2. The DNA-dependent RNA-polymerase activity of isolated rat-liver nuclei was lowered by thyroidectomy and stimulated by the administration of tri-iodo-l-thyronine or l-thyroxine (2-25mug./100g. body wt.) to both normal and thyroidectomized rats. In thyroidectomized rats, the activity of the Mg(2+)-activated RNA-polymerase reaction (for which the product is mainly ribosomal type of RNA) was stimulated at 10-12hr. after a single injection of tri-iodothyronine, reaching a peak value of 60-90% stimulation at 45hr. after hormone administration. The Mn(2+)/ammonium sulphate-activated RNA-polymerase reaction (for which the RNA product is more DNA-like) was not affected for 24hr. after hormone administration but stimulated by 30-40% at 45hr. The response of both RNA-polymerase reactions to the hormone in vivo paralleled the physiological response but the enzyme was not stimulated by the addition in vitro of the hormone to isolated nuclei. 3. Within 3-4hr. after tri-iodothyronine administration to thyroidectomized rats, the specific activity of rapidly labelled nuclear RNA, after a 10min. pulse of [6-(14)C]orotic acid, was 30-40% greater than the control values, the stimulation reaching 100 and 200% at 11 and 16hr. respectively after hormone administration. Longer exposures to [6-(14)C]orotic acid and [(32)P]phosphate showed that the hormone accelerated the synthesis of mitochondrial, microsomal (or ribosomal) and soluble RNA. The greater part of the labelled nuclear RNA was of the ribosomal type. The hormone-induced increases in the incorporation of radioactive precursors into RNA were not preceded, but followed, by enhanced uptake of the precursor. There was no change, per g. of liver, of DNA, nuclear RNA or soluble RNA, but there was a 40-60% increase in the amount of ribosomal RNA between 35 and 45hr. after a single injection of tri-iodothyronine to thyroidectomized rats. 4. Coinciding with the increase in ribosomal RNA after hormone administration was an increase in the average size and amount of polyribosomes. The newly formed ribonucleoprotein particles, or messenger RNA attached to them, or both, were more firmly bound to microsomal membranes after hormone treatment. 5. Polyuridylic acid caused a bigger stimulation of incorporation of [(14)C]phenyl-alanine by ribonucleoprotein particles, but not by microsomes, from thyroidectomized rats as compared with preparations from normal animals. The response of ribonucleoprotein particles to polyuridylic acid was lowered after tri-iodothyronine treatment of thyroidectomized rats. 6. Actinomycin D, 5-fluorouracil, puromycin and cycloheximide caused a 70-100% inhibition of the stimulatory effect of l-thyroxine and tri-iodo-l-thyronine on basal metabolic rate and growth rate in both normal and thyroidectomized animals. Administration of actinomycin D also abolished the stimulation of RNA polymerase by tri-iodothyronine. 7. It is concluded that regulation of nuclear and ribosomal RNA synthesis is an essential step leading to the biological action of thyroid hormones and that the formation of new ribosomes is an important aspect of the control of cytoplasmic protein synthesis by these hormones.     

Binding and translation of turnip-yellow-mosaic-virus ribonucleic acid by skeletal-muscle ribosomes from normal and diabetic rats

Ribosomes from skeletal muscle of diabetic rats were less active than normal ribosomes in protein synthesis directed by turnip-yellow-mosaic-virus RNA. The proportion of ribosomes from muscle of diabetic rats capable of binding turnip-yellow-mosaic-virus RNA was greater than normal, but there was no difference in the equilibrium constants for the binding reaction. The turnip-yellow-mosaic-virus RNA was bound preferentially to the small (40S) ribosomal subunit, whereas the decrease due to diabetes in its translation was associated with the large (60S) subunit. Thus the diminished capacity of ribosomes from muscle of diabetic rats to translate turnip-yellow-mosaic-virus RNA was not the result of decreased binding of the template.     

Protein synthesis in liver,skeletal muscle,and brown adipose tissue of rats fed a protein-deficient diet

Feeding protein-deficient diets to rats is known to stimulate diet-induced thermogenesis and activate brown adipose tissue (BAT). The fact that BAT protein content, unlike that of other tissues, is unnaffected by protein deficiency prompted us to measure tissue protein synthesis in vivo in animals maintained on normal- (18.8%) and low- (7.6%) protein (LP) diets. Protein synthesis was depressed in the liver of the LP rats due to a fall in RNA activity, with no change in RNA content, and synthesis was also reduced in skeletal muscle from the LP group, but this was due to decreased RNA content with no change in RNA activity. Conversely, protein synthesis, RNA, DNA, and protein content of interscapular BAT were all unaltered in protein-restricted animals. These data indicate that, unlike liver, skeletal muscle, and whole carcass, BAT protein synthesis is not reduced in protein-restricted rats, and this may be related to activation of thermogenesis in the tissue.     

Reducing plasma HIV RNA improves muscle amino acid metabolism

We reported (Yarasheski KE, Zachwieja JJ, Gischler J, Crowley J, Horgan MM, and Powderly WG. Am J Physiol Endocrinol Metab 275: E577-E583, 1998) that AIDS muscle wasting was associated with an inappropriately low rate of muscle protein synthesis and an elevated glutamine rate of appearance (Ra Gln). We hypothesized that high plasma HIV RNA caused dysregulation of muscle amino acid metabolism. We determined whether a reduction in HIV RNA (> or =1 log) increased muscle protein synthesis rate and reduced R(a) Gln and muscle proteasome activity in 10 men and 1 woman (22-57 yr, 60-108 kg, 17-33 kg muscle) with advanced HIV (CD4 = 0-311 cells/microl; HIV RNA = 10-375 x 10(3) copies/ml). We utilized stable isotope tracer methodologies ([13C]Leu and [15N]Gln) to measure the fractional rate of mixed muscle protein synthesis and plasma Ra Gln in these subjects before and 4 mo after initiating their first or a salvage antiretroviral therapy regimen. After treatment, median CD4 increased (98 vs. 139 cells/microl, P = 0.009) and median HIV RNA was reduced (155,828 vs. 100 copies/ml, P = 0.003). Mixed muscle protein synthesis rate increased (0.062 +/- 0.005 vs. 0.078 +/- 0.006%/h, P = 0.01), Ra Gln decreased (387 +/- 33 vs. 323 +/- 15 micromol.kg fat-free mass(-1).h(-1), P = 0.04), and muscle proteasome chymotrypsin-like catalytic activity was reduced 14% (P = 0.03). Muscle mass was only modestly increased (1 kg, P = not significant). We estimated that, for each 10,000 copies/ml reduction in HIV RNA, approximately 3 g of additional muscle protein are synthesized per day. These findings suggest that reducing HIV RNA increases muscle protein synthesis and reduces muscle proteolysis, but muscle protein synthesis relative to whole body protein synthesis rate is not restored to normal, so muscle mass is not substantially increased.     

Effect of streptozotocin-induced diabetes and insulin treatment on the synthesis of hexokinase II in the skeletal muscle of the rat

The relative rate of synthesis of hexokinase II in the skeletal muscle of the normal, streptozotocin-diabetic, and diabetic insulin-treated rat was determined by the rate of incorporation of [3H]leucine into hexokinase II and the total cytosolic proteins to determine if the rate of hexokinase II synthesis was altered relative to that of the average protein. This relative rate of synthesis of hexokinase II is approximately 1.9 times higher in the normal than in the diabetic rat. The administration of insulin to the diabetic animal increases the rate of hexokinase synthesis to approximately normal levels. An enzyme-linked immunosorbent assay procedure was developed to determine the amount of hexokinase II protein in the skeletal muscle extracts, and immunoprecipitation was utilized to determine the hexokinase II activity. The specific activity of hexokinase II was determined from these analyses. The specific activity of hexokinase II was the same in the skeletal muscle extracts from normal, streptozotocin-diabetic, and diabetic insulin-treated rats. These results suggest that the decrease in muscle hexokinase activity is not caused by the loss of an activator of the enzyme nor by the increased formation of a hexokinase inhibitor in streptozotocin-induced diabetes; rather the decrease in hexokinase II activity reported in diabetic rats relative to normal animals is a result of decreased synthesis coupled to increased degradation in the diabetic relative to the normal animal.     

Increased phenylalanine incorporation in regenerating skeletal muscle grafts

Skeletal muscle regenerates following grafting, but little is known about protein synthesis and its regulation during regeneration. We determined the sequence of changes in protein synthesis in rat extensor digitorum longus (EDL) muscle by the measurement of phenylalanine (Phe) incorporation into muscle protein at various times after grafting. Compared with control EDL, Phe incorporation in grafts doubled in 1 day, was four- to eight-fold greater from days 2 to 10 after grafting, and then subsided. Tissue mass (wet weight) increased rapidly from days 7 to 20 in EDL grafts. The maximal increase in protein synthesis occurred 7-10 days after grafting, whether or not the nerve was left intact. Autoradiography indicated that incorporated radioactivity was associated with regenerating muscle fibers on day 10. Deficiencies of insulin, pituitary or testicular hormones, or chronic in vivo administration of insulin, growth hormone, testosterone, or tri-iodothyronine did not substantially alter the elevation in incorporation of the Phe into muscle protein 10 days after grafting. The breakdown of EDL protein, measured in vitro simultaneously with protein synthesis, was increased five-fold, and overall protein degradation was elevated six-fold 10 days after grafting. These findings indicate that Phe incorporation is rapidly elevated following grafting of the EDL, and that by days 7-10 reflects synthesis in regenerating muscle fibers. The increase in protein synthesis associated with muscle regeneration at this time appears to be independent of innervation and anabolic hormones.     

Changes in the metabolism of muscle RNA in rats given a high protein diet

The effect of a high protein diet (20% casein + D,L-methionine) administered to adult Wistar rats on some aspects of muscle RNA metabolism has been studied. Body weight increased in spite of lower intake. However, gastrocnemius muscle remained unmodified, although protein content increased. Total RNA decreased in the whole muscle although RNA/DNA ratio did not change. Protein synthesis capacity diminished 81% relative to controls in spite the fact that an excessive amount of available amino acids exists. RNA loss might depend on a high catabolism, since acid RNase activity increased over control values. Therefore, it may be concluded that a high protein diet leads to a lower protein synthesis capacity through an elevated RNA breakdown.     

The stimulation by treatment in vivo with tri-iodothyronine of amino acid incorporation into protein by isolated rat-liver mitochondria

1. Normal and thyroidectomized rats were treated with near-physiological doses of tri-iodothyronine. Liver mitochondria were isolated and incubated with radioactive amino acids. In normal rats tri-iodothyronine caused only a slight stimulation of incorporation into mitochondrial protein, but in thyroidectomized animals the incorporation was doubled. 2. There was a lag period of about 36 hr. after injection and the maximum effect was observed after 2 days. 3. Direct addition of tri-iodothyronine to the incubation medium had no effect on mitochondrial incorporation. 4. The incorporation was not due to bacterial, nuclear, lysosomal or microsomal contamination and the labelled particles had sedimentation properties identical with those of mitochondria, as followed by suitable enzyme markers. 5. Thyroid hormone treatment did not cause any marked alterations in the pattern of labelling of submitochondrial fractions and in all cases the most radioactive protein was in an insoluble lipid-rich fraction. The amino acid compositions of the total mitochondrial protein and the more radioactive lipoprotein were also unaltered. 6. Increases in the content of RNA and various cytochromes per mg. of mitochondrial protein were observed after treatment with tri-iodothyronine. These occurred slightly later than the stimulation of amino acid incorporation. 7. No uncoupling of oxidative phosphorylation was observed and the ATP production per mg. of mitochondrial protein increased. 8. It was concluded that tri-iodothyronine stimulated amino acid incorporation into mitochondrial protein and that the result is consistent with the view that treatment with thyroid hormone results in an enhanced selective synthesis of mitochondrial respiratory units.     

Muscle and liver protein synthesis and degradation in growing rats fed a raw field bean (Vicia faba L.) diet

Body weight gain, food intake, gastrocnemius muscle and liver weight, protein and RNA content, as well as the fractional rates of muscle and liver protein synthesis (ks, according to the method of constant infusion of L-[14C]tyrosine), growth (kg) and degradation (kd), along with RNA activity (g of protein synthesized per day/g RNA) of both organs, were determined in growing male rats fed ad libitum over a period of 10 days on 18.7% protein diets containing either casein (5% of methionine added) (control) or the raw legume field bean (Vicia faba L.) as the sole sources of protein. It has been found that as compared to control rats, those fed the raw legume diet exhibited a significant reduction in the rate of growth, muscle RNA, ks, kg, kd and RNA activity, and a significant increase in liver ks, kd and RNA activity. All differences were statistically significant at least at the 5% level. The possible nature of these findings is discussed.     

Protein and RNA synthesis in the skeletal muscle of hereditary dystrophic mouse.

Protein and RNA contents in muscle of normal and hereditary dystrophic mice C57BL/6J-dy/dy were reexamined on the basis of DNA. It was observed that protein and RNA contents in dystrophic muscle decreased at the early stage of the disease, in disagreement with the reported results on a wet weight basis, in which RNA content in dystrophic muscle had been found to increase. Rates of protein and RNA systhesis in the early stage of the disease were also determined with a concomitant check of the specific activities of free amino acids and free nucleotides. The rates of both protein and RNA synthesis (i.e., specific activities of protein and RNA) were higher in the dystrophic muscle, but when they were expressed on a DNA basis, the total protein synthesis per cell was the same as that of normal muscle and the total RNA synthesis per cell showed a smaller increase in dystrophic muscle. These apparent increases of protein and RNA synthesis were discussed in connection with the decreased protein and RNA contents in the cells of dystrophic muscle. The synthesized RNAs seemed to contain mRNA on the basis of sedimentation character and Millipore filter binding ability. However, no particular RNA was mainly synthesized in dystrophic muscle.     

Tissue-specific changes in protein synthesis associated with seasonal metabolic depression and recovery in the north temperate labrid, Tautogolabrus adspersus

The tissue-specific changes in protein synthesis were tracked in relation to the seasonal metabolic depression in cunner (Tautogolabrus adsperus). In vivo protein synthesis rate and total RNA content were determined in liver, white muscle, brain, heart, and gill during periods of normal activity before metabolic depression, entrance into and during winter dormancy, and during the recovery period. The decrease in water temperature from 8 degrees C to 4 degrees C was accompanied by a 55% depression of protein synthesis in liver, brain, and heart and a 66% depression in gill. Protein synthesis in white muscle fell below detectable levels at this temperature. The depression of protein synthesis is an active process (Q(10) = 6-21 between 8 degrees C and 4 degrees C) that occurs in advance of the behavioral and physiological depression at the whole animal level. Protein synthesis was maintained at these depressed levels in white muscle, brain, heart, and gill until water temperature returned to 4 degrees C in the spring. Liver underwent a hyperactivation in the synthesis of proteins at 0 degrees C, which may be linked to antifreeze production. During the recovery period, a hyperactivation of protein synthesis occurred in white muscle, which is suggestive of compensatory growth, as well as in heart and liver, which is considered to be linked to increased activity and feeding. Seasonal changes in total RNA content demonstrate the depression of protein synthesis with decreasing temperature to be closely associated with translational capacity, but the stimulation of protein synthesis during recovery appears to be associated with increased translational efficiency.     

Myofibrillar protein synthesis in myostatin-deficient mice

Either increased protein synthesis or prolonged protein half-life is necessary to support the excessive muscle growth and maintenance of enlarged muscles in myostatin-deficient mice. This issue was addressed by determining in vivo rates of myofibrillar protein synthesis in mice with constitutive myostatin deficiency (Mstn(DeltaE3/DeltaE3)) or normal myostatin expression (Mstn(+/+)) by measuring tracer incorporation after a systemic flooding dose of l-[ring-(2)H(5)]phenylalanine. At 5-6 wk of age, Mstn(DeltaE3/DeltaE3) mice had increased muscle mass (40%), fractional rates of myofibrillar synthesis (14%), and protein synthesis per whole muscle (60%) relative to Mstn(+/+) mice. With maturation, fractional rates of synthesis declined >50% in parallel with decreased DNA and RNA [total, 28S rRNA, and poly(A) RNA] concentrations in muscle. At 6 mo of age, Mstn(DeltaE3/DeltaE3) mice had even greater increases in muscle mass (90%) and myofibrillar synthesis per muscle (85%) relative to Mstn(+/+) mice, but the fractional rate of synthesis was normal. Estimated myofibrillar protein half-life was not affected by myostatin deficiency. Muscle DNA concentrations were reduced in both young and mature Mstn(DeltaE3/DeltaE3) mice, whereas RNA concentrations were normal, so the ratio of RNA to DNA was approximately 30% greater than normal in Mstn(DeltaE3/DeltaE3) mice. Thus the increased protein synthesis and RNA content per muscle in myostatin-deficient mice cannot be explained entirely by an increased number of myonuclei.     

Turnover of muscle protein in the fowl (Gallus domesticus). Rates of protein synthesis in fast and slow skeletal, cardiac and smooth muscle of the adult fowl.

Rates of protein synthesis in skeletal, cardiac and smooth muscle of fully grown fowl (Gallus domesticus) were determined in vivo by means of the constant infusion method using [14C]proline. In the anterior latissimus dorsi muscle, containing predominantly slow fibres, the average synthesis rate of non-collagen muscle proteins was 17.0 +/- 3.1% per day, a value higher than that obtained for cardiac muscle (13.8 +/- 1.3% per day) and for smooth muscle of the gizzard (12.0 +/- 1.9% per day). In the posterior latissimus dorsi muscle, containing predominantly fast fibres, synthesis rates were much lower (6.9 +/- 1.8% per day). In each case these average rates for the non-collagen protein were similar to the average rate for the sarcoplasmic and myofibrillar protein fractions. The RNA concentration of these four muscles showed that relative rates of protein synthesis were determined mainly by the relative RNA concentrations. The rate of protein synthesis per unit of DNA (the DNA activity) was similar in the two skeletal muscles, but somewhat lower in cardiac muscle and gizzard, possibly reflecting the larger proportion of less active cell types in these two muscles. These quantitative aspects of protein turnover in the two skeletal muscles are discussed in terms of the determination of ultimate size of the DNA unit, and in relation to muscle ultrastructure.     

Scaling effects on metabolism of a teleost

Scaling effects on citrate synthase (CS), glucose-6-phosphate dehydrogenase (G6-PDH), RNA. RNA/DNA ratio and protein contents of brain, liver and skeletal muscle were studied in a teleost, Clarias batrachus. The activity of white skeletal muscle CS decreased significantly as a function of increasing body mass of the fish. It shows that the fulfilment of energy demand in white skeletal muscle is not dependent on aerobic metabolism. The activity of liver G6-PDH decreased with the increasing body mass showing reduction in NADPH generation for lipogenic activity. However, increase in G6-PDH activity showed enhancement in reductive synthesis in skeletal muscle of the larger-sized individuals. A positive scaling of RNA, RNA/DNA ratio and protein contents reflects changes in macromolecular turnover for ATP-supplying enzymes and proteins.     

Effect of diabetes on guanine nucleotide exchange factor activity in skeletal muscle and heart

The present study examined the effects of diabetes and insulin treatment of diabetic rats on the activity of the protein synthesis initiation factor, the guanine nucleotide exchange factor. In extracts from gastrocnemius and psoas muscles from two-day diabetic rats, guanine nucleotide exchange factor activity was reduced to 80% and 67% of control values, respectively. Insulin treatment (2 h) restored guanine nucleotide exchange factor activity to control values in both muscles. In contrast, guanine nucleotide exchange factor activity was unchanged in extracts from either soleus muscle or heart from diabetic rats compared to controls. Also, insulin treatment did not increase guanine nucleotide exchange factor activity in extracts from soleus and heart. The results suggest that the diabetes-induced impairment in peptide-chain initiation in fast-twitch skeletal muscle (i.e. gastrocnemius and psoas) is related to an inhibition of guanine nucleotide exchange factor activity and that slow-twitch muscle is spared from the effect on initiation due to the preservation of guanine nucleotide exchange factor activity.     

Insulinomimetic effects of kaempferitrin on glycaemia and on 14C-glucose uptake in rat soleus muscle

Bauhinia forficata is one of the Bauhinia species mostly used as an antidiabetic herbal remedy in Brazil. Kaempferitrin (kaempferol-3,7-O-(alpha)-L-dirhamnoside) is the predominant flavonol glycoside found in the B. forficata leaves. The aim of the present work was to study the long-term effect of kaempferitrin on glycaemia in diabetic rats, as well as the in vitro effect of this compound on 14C-D-glucose uptake and 14C-leucine incorporation into protein in normal rat soleus muscle. Kaempferitrin was found to have an acute lowering effect on blood glucose in diabetic rats and to stimulate the glucose uptake percentile, as efficiently as insulin in muscle from normal rats. This compound did not have any effect on glucosuria or on protein synthesis in muscle from normal and diabetic animals. However, the protein synthesis in the kaempferitrin-treated groups was maintained at the same level as the respective controls. Thus, the hypoglycaemic effect and the prompt efficiency of the kaempferitrin in stimulating [U-14C]-2-deoxi-D-glucose uptake in muscle -constitute the first evidence to indicate that the acute effect of this compound on blood glucose lowering may occur as a consequence of the altered intrinsic activity of the glucose transporter (Vmax or glucose transporters translocation?) not involving directly the synthesis of new carriers.